What does a narcissistic flying reptile that loves the taste of crispy dwarves have in common with a beetle that shoots hot, caustic liquid from its butt? More than you think.

Credit: Warner Bros

A few weeks ago, audiences were finally treated to the Cumberbatch-infused reptilian villain from J.R.R. Tolkien’s classic The Hobbit. Smaug (pronounced and interpreted as if you smashed together “smug” and “smog”) is a terrible dragon that long ago forced a population of dwarves from under a mountain. He laid claim to all their treasures. He burned all their homes. The titular character of the book is then tasked with helping a company of these displaced dwarves take back the mountain from the beast. It wouldn’t be easy—the most common descriptor of a dragon is “fire-breathing,” after all. But unlike other aspects of the book and now the film that are wholly magic, Smaug’s burning breath is actually one of the least magical, and can be wrangled into plausibility. Doing so involves looking inside a beetle’s butt, a Boy Scout’s satchel, and a bird’s throat.

Even though they don’t exist, dragons, like all other real organisms, have evolved over time. They weren’t always so huge. Dragons were once the size of cattle in popular depictions. And some of them didn’t have wings, or breathe fire. Today’s dragons are uniquely terrible lizards—massive, spiked, voracious, flame-spewing beasts. If even mythical beasts evolve, how would a real dragon evolve its most recognizable ability?

First, a dragon needs fuel.

This aspect of dragon fire is the easiest to imagine. In fact, you are probably producing dragon fuel as you read this sentence. Methane—a highly flammable gas that is produced naturally by bacteria in the gut—is constantly bubbling up in your stomach as microbes munch on your food. With a large stomach or even a separate organ to house this gas, a dragon could easily eat enough food to produce a large amount of methane.

If methane isn’t the fuel, another more exotic liquid might be. Take the bombardier beetle. This incredible insect evolved (yes, evolved) a way to harness the chaos of chemical reactions in a defense mechanism. When under threat, the beetle excretes two chemicals from two separate reservoirs that mix in a third, producing a very hot liquid and the gas needed to propel it into the face of some would-be predator.

When two liquids come together, react, and spontaneously combust, they are called “hypergolic.” The bombardier beetle isn’t the only organism that takes advantage of hypergolic chemicals; we use them in rocket fuel. (You can see a nice small-scale demonstration of a hypergolic reaction here.) A dragon could do the same. It wouldn’t be the first fiction animal to do so either. Who can forget (or maybe remember?) the giant, fire-spewing “tanker” bug from the ironically classic sci-fi movie Starship Troopers?

If a dragon convergently evolved chemicals that combust upon mixing, like the explosive bombardier beetle, the reaction it harnessed could result in fire…terrible, terrible, fire. But these chemicals aren’t cheap, biologically speaking. A dragon would have to make a large biological investment to produce them. That would at least be consistent with dragons’ voracious appetite for dwarves, men, and livestock—the winged beasts need to produce more rocket fuel.

The next step in fire breathing is the spark.

Before you see a dragon’s flame you see the teeth. Terrifying spears and stake knives that click and clamor inside gigantic mouths—giant flints. Some dragon lore speculates that dragons, like modern birds, ingested rocks and stones to aid in digestion. Over time, stories say, the minerals would coat dragon teeth. Or maybe the dragons could hold some minerals in their mouths. Either way, quickly biting down on these minerals could produce a spark. Like a Boy Scout’s trusty flint, clicking dragon teeth would provide the ignition for either a glut of methane gas or a gush of hypergolic liquids (if needed).

Another possible spark could come from more detailed physics. If dragon teeth had piezoelectric properties—where mechanical stress produces small jolts of electricity—a combination of methane exhalation and teeth grinding could light the fire. Or maybe the crushed stones and minerals could vaporize in the air ahead of the methane and combust, as metals do on helicopter rotors in the Kopp-Etchells Effect. Perhaps the dragons could expel the liquids or gases so quickly from their bellies that static ignition would occur. (When cleaning out supertankers, for example, all the flammable vapors first must be vented. The high-pressure water jets used in cleaning can generate sparks that ignite the gas.) Could a dragon evolve an organ that produces its own spark like a kaiju from Pacific Rim? Science has many sources of ignition to choose from, it’s just a matter of what the fiction allows.

Dragons are basically our pipe-dreams of what birds would be if they still looked liked ancient dinosaurs but followed evolution’s flight plan. Dragons’ similarities with birds (themselves in fact dinosaurs) could provide the last critical link to flame flinging. With multiple stomachs aiding digestion, birds—and by extension, dragons—could evolve a specialized sack for storing either methane or combustible chemicals. Birds also eat stones and rocks to break up tough material in these stomachs, so Smaug munching on minerals isn’t that far-fetched either.

There are still a few problems. Dragons are huge, undoubtedly heavy creatures that would likely rip their own wings to shreds when attempting to stay airborne. Also, holding both a large amount of methane or hypergolic chemical internally is explosively problematic. Breathing out fire is a problem in and of itself. A dragon would need specialized tissues in the mouth to deal with the incredible heat, and lungs large enough to force the flames a significant distance (unless it was a dragon from Skyrim, which specializes in powerful blasts of voice). I imagine that a more scientifically plausible dragon wouldn’t be the slender, monitor lizard-like monstrosity we see in the latest Hobbit film and more like a flightless, bloated flame-thrower.

What does this mean for The Hobbit, a universe already filled with magic? Nothing at all. The film has its own problems to deal with. Nonetheless, I found Smaug’s fire breath to be one of the least distressing suspensions of reality. Whether a “true” fire-breathing dragon is filled with flint, gas, or rocket fuel, one thing is for sure: Where there is Smaug, there is fire.

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About Kyle Hill

Kyle Hill is a science writer and communicator who specializes in finding the secret science in your favorite fandom. His work has appeared in Wired, The Boston Globe, Scientific American, Popular Science, Slate, and more. He is a TV correspondent for Al Jazeera America's science and technology show TechKnow and a columnist for Skeptical Inquirer magazine.
Find his stream of nerdery on Twitter: @Sci_Phile
Email him at sciencebasedlife [at] gmail [dot] com.